It is common practice to employ an electronic data buffer between intermediate data units for enhancing data exchanges between the units. It is particularly advantageous to provide an electronic buffer between an electro-mechanical data processing device and a completely electronic data processing unit. An example of such an arrangement is a magnetic tape drive coupled to a central processing unit (CPU) which is a host or host processor to the tape drive. The idea is to mask or hide the relatively slow electro-mechanical device actions from the electronic data processing unit. An example of such masking is shown by R. A. Gregory, et al U.S. Pat. No. 2,960,683. An electronic data buffer, storing a block of data, was interposed beween a CPU and a magnetic tape drive of the vacuum column type. This buffer-masking arrangement has been extended conceptually to include so-called cache memories which completely, at least in theory, mask one unit from another unit. Such cache memories are generally employed between the main memory and a processing unit of a CPU. Such "caching" has also been extended to peripheral subsystems such as in the IBM 3850 Mass Storage System available from International Business Machines Corporation, Armonk, N.Y. In the IBM 3850 system, a plurality of direct access-storage devices (DASD) completely mask operation of an automatic magnetic tape library. While the IBM 3850 system provides an enormous storage subsystem, not all storage subsystems need to reach the performance and storage capability of the IBM 3850 system. Detailed descriptions of the IBM 3850 MSS are found in publications available from International Business Machines Corporation, Armonk, N.Y. identified as GA32-0028, "Introduction to the IBM 3850 Mass Storage System (MSS)" 1974; GA32-0035, "IBM 3850 Mass Storage System (MSS) Principles of Operation; Theory" 1978; GA32-0030, "IBM 3850 Mass Storage System (MSS) Installation Guide" 1977; and GA32-0029, "IBM 3850 Mass Storage System (MSS) Principles of Operation" 1975.
Accordingly, it is desired to provide a data buffer in a tape storage subsystem between a plurality of tape drives and a plurality of hosts via a plurality of input/output channel connections. The buffer is to be shared by the plurality of tape drives such that the number of buffer storage segments of the buffer is less than the number of tape drives. It is also provided that data is transferred from a magnetic tape drive to its allocated buffer segment in anticipation of a request from a host 15 prefetch. This so-called prefetching can result in data being transferred into the buffer from a tape drive and that host or other activities then occur requiring that the prefetched data not be used. At this time, corrective action has to be taken in the subsystem for maximizing performance. It is a purpose of the present invention to provide that corrective action at minimum cost while maintaining a maximal performance level.